Throttle bodies for modulating the flow of gases are well known in, for example, the automotive art. A typical throttle valve comprises a valve body having a generally cylindrical bore for flow of gas, the bore being intersected generally diametrically by a rotatable shaft supporting a generally planar valve head capable of closing or opening the bore when the shaft is rotated to extremes, and also of modulating flow through the bore at rotational positions in between. The valve head may be elliptical in plan view such that the valve head can close the bore when rotated to an angle less than orthogonal to the axis of the bore, to prevent sticking of the valve head in the bore at the occluded position.
In some applications, for example, in throttling the exhaust gas stream of a diesel engine as part of an assembly for exhaust gas recirculation (EGR), two parallel-mounted throttle valves are required which are commonly provided as dual bores in a common valve body with a common shaft intersecting both bores and a valve head disposed on the shaft in each bore, such that the flow performance of the two valves is identical with rotation of the shaft. Of course, the bores may be of different diameters or cross-sectional shapes as required for any particular application.
In known throttle valves, the shaft is retained in the valve body by shaft axial restraining means outboard of the gas flow bore or bores and near an end of the shaft. This construction is known in the art for both single-bore and multiple bore throttle valves.
Use conditions for throttle valves, especially temperatures of use, may be very different from the ambient conditions under which they are manufactured, and valve components may expand or contract accordingly. In throttle valves constructed in accordance with the prior art, thermal expansion of the valve shaft is unidirectional from the shaft restraining means, and is cumulative therefrom. For single-bore valves, the misfit of the valve head in the bore caused by thermal changes can be acceptable because the shaft length is short relative to the diameter of the bore. However, for some dual-bore valves, and especially for diesel EGR valves which may be required to function properly between start-up temperatures of -30.degree. C. or lower and equilibrium temperatures of 800.degree. C. or higher, thermal shaft length changes can be great enough to cause the valve head farther from the restraining means to bind in its bore and fail to modulate the flow of gas therein as intended. Such failure, of course, causes an identical linked failure in the near bore as well. For triple-bore valves, the problem becomes proportionally more severe.
What is needed is a improved means for restraining end play of a valve shaft in a multiple-bore valve such that thermal expansion of the shaft from the point of restraint is insufficient to prevent the valve from proper function.